The present invention is related to the operation of steam turbine-generators and provides apparatus for dealing with turbine overspeed following a sudden loss of load and for dealing with the application of preheat to certain sections of the turbine prior to application of significant load.
In the operation of a steam driven turbine-generator, decreases in electrical load on the generator tend to cause an increase in rotational speed. With a sudden and substantially complete loss of generator load, such as occurs, for example, with a circuit breaker tripout, there is some potential for increasing the turbine speed even to destructive levels. This prospect is, of course, carefully guarded against and protective overspeed control means have been developed and incorporated into turbine control systems to rapidly close the steam valves and shut off the supply of motive fluid as an overspeed condition is detected. In an extreme condition the turbine is "tripped" automatically by the control system, a condition requiring operator intervention before steam is again admitted to the turbine.
Following an overspeed closure of the steam valves, there is a significant additional speed rise attributable to steam retained within the turbine stages, shells, inlet passages, various crossovers, extraction lines, and so forth. This steam exhausts itself through lower pressure sections of the turbine and, in the absence of a load to sustain, the energy of the steam is spent by increasing the turbine speed. While this may be regarded as a momentary or transient condition, overspeed due to these "entrained steam energies" must be kept within reasonable bounds for several important reasons.
Among these, three are particularly noteworthy. First, the speed rise must be limited to a level at which the resulting centrifugal stresses on the rotor do not significantly detract from its operating life. Second, it is desirable to maintain the speed rise below the value at which automatic tripping takes place, so that the turbine remains under control of the speed governor, ready to assume load to satisfy the requirements of the power system. Third, for those situations in which local auxiliary equipment remains electrically tied to the generator, it is important to limit the speed rise to a value that is not detrimental to such auxiliary equipment.
As an additional consideration, the art of steam turbine and generator design has progressed such that the relationship between maximum power output and the moment of inertia of the rotating component has changed in a direction which makes it even more difficult to keep the speed rise resulting from "entrained steam energies" within reasonable bounds.
Accordingly, it is an object of the present invention to provide control apparatus by which turbine overspeed is more closely controlled following a sudden loss in the load on a turbine-generator.
Significantly, the apparatus of the present invention also provides a solution to another problem of long standing in the operation of a turbine-generator. That is, in the higher pressure sections of a steam turbine the shell and rotor components are constructed of alloys that have excellent strength when operating at high temperatures but which must be operated above a minimum temperature of about 300.degree. F. to render them properly ductile. In the past, to preheat the high temperature portions of a turbine enough to attain this threshold temperature, a complex and lengthy procedure has been required before the turbine could be put to productive use.
In this regard, it is yet another object of the present invention to provide means by which the turbine preheating period can be reduced and replaced by a less complex, shorter preheating process.